The present invention relates to a joining system head for attachment to a movable frame, in particular to a robot, having                a holding means for an element to be joined to a part, and        a joining drive means to move the holding means along a joining direction for joining.The present invention relates further to a joining system having a robot movable on at least two coordinate axes and a joining system head attached to the robot. Lastly, the present invention relates to a method of feeding elements from a stationary unit to a movable joining system head and joining said elements to parts by means of the joining system head.        
Such a joining system head, such a joining system and such a method of feeding and joining elements by means of a joining system head are generally known. The term ‘joining’ in the present context is intended to refer to all ways of connecting elements to parts, in particular connections of metal elements to metal parts, for example by bonding, forming, as for example riveting, or by union of matter, as for example welding, including short-time arc welding. Short-time arc welding is often referred to as bolt welding, even though it is not exclusively bolts that are welded. A current system of bolt welding in industrial use, in combination with a robot, is known in the brochure “Neue TUCKER Technologie. Bolzenschweiβen mit System!,” Emhart TUCKER, 9/99.
Bolt welding finds application chiefly, but not exclusively, in vehicular technology. Here, metal elements such as metal bolts, with or without threads, eyes, nuts etc., are welded onto the sheet metal of the bodywork. The metal elements then serve as anchors, or fastening elements, to fix for example interior fittings, lines and the like to the sheet metal of the body. At the joining system head, disclosed in the above-mentioned Emhart TUCKER publication, the joining drive means is configured either as a linear electric motor or as a combination of a lift magnet and a spring.
The holding means is constituted by a one-piece tongs elastically expandable in radial direction. The elements are as a rule welding bolts comprising a head having a somewhat larger diameter than the shank of the bolt. In the known system, the bolts are fed to the welding head by way of suitable feeding conduits by means of compressed air. The bolts are thus fed ‘head first’ into the tongs from behind. Ordinarily the bolt will strike the tongs from the inside, but without passing through it. A loading pin provided coaxial with the tongs is then actuated to propel the bolt thus fed through the tongs. The tongs are elastically expanded radially when the head of the bolt passes through. Then the tongs snap closed elastically around the shank of the bolt and hold it fast in the position determined by the travel of the pin.
The joining drive means in the form of a linear motor (or lift magnet/spring combination) has a travel of a few millimeters. Also, the welding head is fixed at the end of an arm of the robot, usually by way of a pneumatic or hydraulic carriage. That is, the entire welding head is movable in a direction parallel to the welding axis by means of the carriage, which has a considerably greater travel than the linear motor. The welding head further comprises a control means to control the linear motor and the loading pin, provided spatially separate from the welding head, more specifically in a stationary feeder.
To perform a welding operation, first the robot is programmed so that it travels into a predetermined position in which the carriage and linear motor axes are perpendicular to the sheet metal onto which the bolt is to be welded. The bolt is prestressed so that it protrudes vis-à-vis a supporting foot. Then the carriage is actuated until the foot meets the sheet metal. The bolt held in the holding means then rests in contact with the sheet metal. Next comes a determination of the zero line of the holding means with respect to the sheet metal. Alternatively, however, there are methods of zero line determination that dispense with the supporting foot.
Then, in the case of welding with supporting foot, an electric pre-current is switched on, passing through the bolt and the part. The bolt is then lifted relative to the part by means of the linear motor (lifting means). An electric arc is set up. Then a switch is made to the welding current. By the high welding current, the opposed faces of bolt and part begin to be fused. The bolt is then lowered onto the part again, so that the respective melts will mingle. Upon attainment of the part and the short circuit of the arc, or just before, the welding current is switched off. The entire melt solidifies and the welded connection is complete.
Now the welding head is drawn off from the welded-on bolt, using the carriage. The carriage is necessary because, among other reasons, the drawing-off motion must take place exactly on the centerline of the welded-on bolt. Otherwise, owing to the one-piece tongs, there would be danger of damage to the bolt and/or the tongs. The robot arm alone is not capable of such a precise linear motion in an arbitrary direction of space. For owing to the superposition of the simultaneous regulation of several components of robot arm motion, as required for this purpose, such linear motions can be executed by the robot with a certain amount of undulation only. The known welding head comprises a comparatively great axial extent. Since moreover the welding head must be drawn off from the bolt in axial direction, use of the welding head in places of difficult access is possible only within limits.
Then there are developments for employing robot technology to feed the bolt. Here a separate pick-up takes pre-sorted bolts and brings them to the welding location. This is disclosed in “Bolzenschweiβen. Grundlagen und Anwendung” by Trillmich, Welz, Fachbuchreihe Schweiβtechnik, DVS Verlag, 1997, Chapter 9.3. It is there explained that this technology lends itself especially to headed bolts that, because of their size and shape, cannot be blown through hoses. This type is referred to as the “pick-up system.”
Further, a welding head by the firm of Nelson has been disclosed, in which a lift device moves a carrier projecting laterally arm-like up and down. At the terminal portion of the carrier, a holding means with tongs is rigidly mounted. The bolts are fed, as in the case of the TUCKER welding head described above, to the tongs from behind, by means of a compressed air hose extending through the carrier. The end portion of the carrier with holding device fixed thereto is more readily positioned at inaccessible locations. The lift device to move the projecting arm and the pertinent control means are arranged in the initial portion of the carrier.
Against this background, the object of the invention consists in specifying an improved joining system head, an improved joining system and an improved method of feeding and joining fed elements. This object is accomplished, in the case of the joining system head initially mentioned, in that the holding means is mounted rotatable at the joining system head about an axis extending transverse to the joining direction.
The joining system head according to the invention represents a completely novel concept. For joining, in particular for bolt welding, the joining operation of the prior art always takes place in a linear motion. In the prior art, consequently, it was the practice to mount the holding means slidable at least along a linear axis. For example, it is known that the joining system head may be mounted bodily on a carriage which in turn is fixed to the robot.
Owing to the rotatable mounting of the holding means on the joining system head, it is now possible to move the holding means along a circular or circular arc path. This creates the prerequisite for a number of fundamental changes in past concepts of joining system heads. The rotatability of the holding means is comparatively simple to realize as a matter of design. In particular, it is possible by means of the robot to turn the holding means on the circular path in order to reach various welding positions quickly and without extensive motion routines, for example a welding position for welding in vertical direction downward and then an overhead welding operation. The turnability as an additional degree of freedom at the joining system head is sufficient for many applications. Considering that also the carrier itself is rotatable about its longitudinal axis by means of the robot as a rule, and positionable at will in space, joining operations can be performed at very inaccessible locations indeed. The interference edge profile of the joining tool is here determined by the required radius of swing. In the joining system according to the invention, the above object is accomplished in that a joining system head according to the invention is attached to the robot.
The method according to the invention for feeding elements from a stationary unit to a movable joining system head and for joining said elements to parts by means of the joining system head, said joining system head comprising a holding means for an element, mounted rotatable about an axis extending transverse to the joining direction, includes the steps of feeding an element from the stationary unit to the transfer station at the joining system head, rotating the holding means towards the transfer station, taking over an element from the transfer station into the holding means, and joining the element taken over to the part. In the method of the invention, accordingly, there is a fundamental departure from the idea of feeding elements from the stationary unit directly to the holding means. Instead, the elements are fed to a transfer station at the joining system head, and the holding means is rotated towards the transfer station, to pick up the elements. The holding means thus ‘fetches’ the elements from the transfer station in each instance. As a result, there is an uncoupling between the feeding means comprised by the transfer station and the holding means. This is a prerequisite for a number of advantages about to be illustrated in detail. The object, then, has been wholly accomplished.
It is of especial advantage if the holding means and the joining drive means are mounted rotatable about the axis as a joining tool. In this embodiment, the holding means and the joining drive means form a rotatable unit of small dimensions. This is true especially if a control means to control the joining drive means is mounted at the welding head, but spatially separate from the joining tool. The joining tool can consequently be made with small dimensions and little relevant edge interference.
Here it is especially preferred if the joining tool is mounted rotatable about the axis at an end portion of a projecting elongated carrier. Owing to the arrangement of a joining tool of small dimensions at the end portion of an elongated carrier, it is possible to bring the joining tool to places difficult of access. Here no transmission of a lifting motion over long distances (no boom or the like) is required. Therefore the positioning and the actual joining or welding operation itself can be performed locally with high precision. At the same time, it is especially advantageous if the control means is provided in an initial portion of the carrier. The joining tool of small dimensions can then be brought to inaccessible locations through openings.
In an especially preferred embodiment, the elongated carrier comprises two arms running parallel, between which the joining tool is rotatably mounted. This embodiment has the advantage, firstly, that the mounting of the joining tool can be accomplished with high spatial precision. Besides, the space remaining between the arms of the carrier can be utilized for other functional units. These units as well as the joining tool are moreover protected between the arms of the carrier.
It is of especial advantage also if the axis of rotation is oriented transverse to the longitudinal axis of the carrier. In this embodiment, it is advantageously brought about that the circular path of the holding means can extend beyond the foremost ends of the carrier. Consequently, the carrier can be of comparatively short configuration. Secondly, it is brought about that the holding means can be swung as far as a midportion of the carrier, and can therefore be brought all the way to other functional units.
Over all, then, it is of advantage firstly if the joining system head comprises a feeding means with transfer station for the feeding of elements and if a loading drive means is designed to rotate the holding means and/or the joining tool all the way to the transfer station. Thus the elements are not fed, as in the prior art, all the way to the holding means. Rather, the feeding of the elements at first takes place only as far as the transfer station. Hence this step of the feed can take place while the joining-welding head itself is joining an already fed element to a part. This parallel processing serves to permit shorter periods over all. It is especially preferred if the transfer station is fixed to the carrier. Provided the transfer station is arranged on the elongated carrier, a fixed relative position of the transfer station can be achieved in relation to the holding means or the joining tool. Besides, it is advantageous that the cross section of the carrier is smaller as a rule than the cross section of the joining tool or the holding means, so that space is available for the transfer station.
According to one embodiment, the loading drive means comprises a rotary motor arranged at the end portion of the carrier. In this embodiment, a precise control of the joining tool can be achieved, with good response behavior.
In an alternative embodiment, the loading drive means comprises a rotary motor arranged in the initial portion of the holder and a gear to transmit the motions of the motor to the holding means. In this embodiment, an improved interference edge clearance results, since the interference-relevant end portion of the carrier has no motor of its own to move the holding means and/or the joining tool. Rather, the comparatively bulky motor is arranged in the initial portion of the carrier and transmits its motions to the holding means and/or the joining tool by way of a transmission. Also, a rotary motor will serve to execute motions with precision and high responsiveness.
It is especially preferred if the transmission is a transmission with tension means. The transmission with tension means will permit comparatively long distances between the initial portion of the carrier on the one hand and the final portion of the carrier on the other hand by comparatively simple design means.
In general, in a preferred embodiment, provision is made for the loading drive means and the joining drive means to consist of a single rotary drive means. In this embodiment, the rotatability of the holding means is used not only to swing the holding means all the way to a transfer station to ‘fetch’ an element. Rather, the holding means is moved to join a held element, not in a direction perpendicular to the axis of rotation, but along a circle around the axis of rotation. This embodiment has the special advantage that an axially prolonged linear drive in the region of the holding means, in particular the end region of the elongated carrier, is not required. Rather, the rotary drive means constituting the loading drive means and the joining drive means may be provided for example in the initial portion of the carrier, and their motions can be transmitted to the holding means in the end portion of the carrier by way of a transmission with tension means. In this embodiment, a sort of ‘reduced’ joining tool is formed at the anterior end portion of the carrier, consisting basically of the holding means alone. In this embodiment, consequently, an especially low interference edge relevance results, and hence the possibility of performing joining operations even in especially inaccessible locations.
In an alternative embodiment, the joining drive means comprises a linear drive means instead. In this embodiment, the holding means is consequently set in rectilinear motion for joining in conventional manner. The rotatability of the holding means about the axis of rotation is then preferably employed by means of development of the loading drive means to rotate the holding means or the joining tool into any welding position and/or ‘fetch’ elements from a transfer station of the feeding means.
Provided the linear drive means comprises a linear electric motor, only comparatively few lines are required for control. The holding means may then be regulated in either lift direction. In this embodiment, it is of especial advantage if the longitudinal axis of the joining drive means and the longitudinal axis of the holding means are spaced apart parallel to each other. Here it is possible to position the holding means so that even welding positions close to edges are attainable. The distance of the longitudinal axes may be within the range of a few centimeters, just enough to shift the holding means out of the joining drive direction projected into the joining direction.
In general, provision is made, in a preferred embodiment, for the holding means to comprise a plurality of jaws arranged distributed around the longitudinal axis of the holding means and movable away from each other so as to hold or release one element in each instance. It is especially preferred if the holding means comprises two jaws. The term ‘jaws’ is to be understood broadly in the present context. The jaws may for example refer to elongated fingers. With two jaws, rotationally symmetrical or approximately rotationally symmetrical parts in particular can be picked up conveniently and held securely. It is preferred for the jaws to be movable away from each other far enough so that the holding means can release the element by being drawn off from the element obliquely to the joining direction.
This embodiment makes it possible to accomplish the process of ‘running’ the joining system head away from the element joined to the part, by means of the robot alone. Then no carriage is required to establish a completely rectilinear reverse motion. Thus this embodiment also contributes to a small axial extent of the welding head.
However, it is especially preferred if the jaws are movable away from each other far enough so that the holding means can release the element by being swung away from the element about the axis of rotation. In this embodiment, the jaws can be moved far enough away from each other so that the joining tool need not be run away in the joining direction. Rather, it is possible to run the joining tool and/or the holding means away after the joining operation transversely, in particular perpendicular to the joining direction, the element passing between the jaws of the holding means. In this embodiment, therefore, no axial motion is required. In this way it is possible to pass the carrier with joining tool and/or holding means arranged at the anterior end portion through even extremely small means, and execute joining operations inside of cavities. The carrier, after the joining position has been reached, can remain positioned almost without change. After the joining operation, the joining tool is swung away transversely to the joining direction, and then the carrier can be run out of the cavity again along its longitudinal axis.
Also, this embodiment makes it possible for the elements to be picked up especially simply from the transfer station. The joining tool in this embodiment is swung in one step so that the holding means is oriented with an element at the transfer station with jaws released. Then the element can be grasped by the jaws and taken out of the transfer station by an ensuing swinging motion. In general, it is here preferred for a jaw actuator to be provided, actively opening and/or closing the jaws. In this embodiment, the jaws are usually configured as rigid fingers. The jaw actuator ensures that the jaws are either actively opened, to release an element, or else actively closed to hold the element.
Alternatively, it is possible for the jaws to be elastically configured or elastically mounted, in such manner that they are passively movable towards and/or away from each other. Here the jaws may either be made of an elastic material, in which case other elastic means may as a rule be dispensed with, or alternatively the jaws may be configured as rigid elements and elastically mounted. It is also possible within the scope of this embodiment for the jaws to be elastically pre-stressed in holding or in releasing direction. In that case, as a rule an actuator is provided that moves the jaws actively in the respective other direction.
In the joining system according to the invention, it is of advantage if a stationary individualizing means conveys individual elements to the feeding means of the joining-welding head. This embodiment serves generally to enhance the degree of automation. Such stationary individualizing and feeding means are known per se in the prior art. They convey individual elements in one step, however, all the way to the holding means, whereas in the joining system according to the invention, a conveyance occurs only as far as the feeding means (transfer station). Thence the holding means ‘fetches’ an element conveyed thither. It will be understood that the features named above and the features yet to be illustrated below may be employed not only in the combination given in each instance, but also in other combinations or by themselves, without departing from the scope of the present invention.